Power consumption is a crucial factor in the deployment of Digital Subscriber Line (DSL) modems. As technology evolves, the power consumption of the digital part of the modem can be reduced significantly by using new processes, such as moving from 0.5 μm semiconductor devices to 0.25 μm and 0.18 μm devices. In contrast to the digital part, the power consumption of the analog part of the modem remains largely unchanged, and thus becomes the most significant factor in overall power consumption.
One approach to reducing the power consumption takes advantage of the large periods of time in which the transmission includes only idle information, such as ATM idle cells. This approach defines a quiescent mode, or Q-mode, which the modem transmitter enters during these idle periods. Possible characteristics for Q-mode operation have been proposed in contributions to the International Telecommunications Union (ITU), such as contribution D.282, by 3Com Corporation, and contribution D.415, by Alcatel. These contributions are incorporated herein by reference.
Q-mode proposals to date have focused on discrete multi-tone (DMT) transmission methods, which are used in Asymmetric DSL (ADSL). These proposals are based mainly on limiting transmission during idle periods to a single tone, preferably the pilot tone that is normally used for frequency tracking. The advantages of the pilot tone method include:    Very low power consumption—transmitting only about {fraction (1/256)} of the total nominal power of the transmitter.    Continuous frequency tracking, so that the transmitter and receiver remain locked to the proper frequency.    Very simple to implement in the transmitter.    Easy to detect at the receiving end.
This solution suffers from a significant problem, however: it creates a non-stationary noise environment for neighboring modems. The spectral profile of crosstalk noise received by other modems (known as far end crosstalk—FEXT—and near end crosstalk—NEXT) is not constant in time. The noise is low during Q-mode operation, because only one carrier is transmitted, but it increases sharply when the modem returns to its normal transmit mode, in which most of the 256 ADSL carriers are used. The sensitivity of ADSL receivers to non-stationary cross talk varies with frequency and with the type of interference (NEXT or FEXT), as described in contribution D.832 to the ITU, by Alcatel, entitled “Effects of Pulsating Crosstalk on Vintage ADSL modems,” which is incorporated herein by reference.
The result of this non-stationary behavior is performance degradation. The ADSL receiver tracks the noise and attempts to adapt to its particular characteristics so as to maximize the noise margin or the data rate. The modem cannot track the sharp changes in the noise floor due to Q-mode transitions, however, and the result is a long burst of errors upon every transition. This error burst may be severe enough to cause the modem to “retrain” for the altered noise conditions. Such retraining is undesirable, because it interrupts modem service for several seconds. Proposed techniques for dealing with non-stationary crosstalk due to Q-mode transitions are described in contributions TD22 and TD26 to Standards Committee TM6 of the European Telecommunications Standards Institute (ETSI), submitted by Alcatel. These contributions are incorporated herein by reference. Implementation of these proposed techniques would require modification and/or reprogramming of currently-deployed modems.
Another approach for reducing transmitter power consumption is to dynamically switch the power supply voltage of the transmitter between two levels. This method does not require any special signal for Q-mode, and actually does not require a Q-mode at all. It is applied to the transmitted signal while the modem operates in its normal data mode. The disadvantage of this method is that it requires very rapid switching of the power supply, typically within less than 2 μsec switching time. Such fast switching is likely to degrade the linearity of the transmitter's line driver. For this reason, the idea of voltage switching has not been adopted in any practical modem. Contribution D.835 to the ITU, by Alcatel, notes that power consumption of an ADSL transmitter can be reduced by static reduction of the line driver supply voltage and bias current. The contribution indicates that there is a need for dynamic power saving methods. It concludes, however, that further study of the degrading effects of non-stationary crosstalk is required in order to define an optimal dynamic power saving technique.